1. Field of the Invention
The present invention relates to a method of manufacturing a GaN substrate, a method of manufacturing an epitaxialwafer, a method of manufacturing a semiconductor device and an epitaxialwafer, and more particularly, it relates to a method of manufacturing a GaN substrate having a c-plane for manufacturing an epitaxialwafer by successively stacking at least two layers including an AlxGa(1-x)N layer and a GaN layer on the c-plane, a method of manufacturing an epitaxialwafer, a method of manufacturing a semiconductor device and an epitaxialwafer.
2. Description of the Background Art
In general, a GaN (gallium nitride) substrate is employed as the substrate for a semiconductor device such as a light-emitting diode (LED) or a laser diode (LD). When GaN having an energy band gap of 3.4 eV and high thermal conductivity is applied to the substrate for the semiconductor device, an electrode can be provided on the back surface of the GaN substrate, and the driving (operating) voltage of the semiconductor device can be reduced.
Such a GaN substrate is manufactured by a method described in K. Motoki et al., “Preparation of Large GaN Substrates”, Materials Science and Engineering B93 (2002), pp. 123-125, for example. This document discloses that the GaN substrate is manufactured through the following steps: First, a buffer layer of GaN having a thickness of 60 μm is formed on a GaAs (gallium arsenide) substrate by HVPE (Hydride Vapor Phase Epitaxy). Thereafter a GaN layer having a thickness of 500 μm is formed on the buffer layer by HVPE. Thereafter the GaAs substrate is removed, and a GaN substrate having a thickness of 495±10 μm is obtained by polishing.
According to the aforementioned document, however, no plurality of GaN substrates are cut out from a bulk GaN single crystal in the thickness direction, and hence a high cost is disadvantageously required for obtaining the GaN substrate.
In order to reduce the cost for each GaN substrate, a technique of manufacturing a GaN substrate by manufacturing an ingot of GaN having a large thickness and cutting out a plurality of GaN substrates from this ingot in the thickness direction is conceivable. If the GaN substrate cut out from the ingot has a thickness of 495±10 μm as described in the aforementioned document, however, this thickness may be excessive, depending on the performance of a warp required to the semiconductor device formed on this GaN substrate. In this case, the cost cannot be sufficiently reduced.
While the GaN substrate is preferably cut out from the ingot with a smaller thickness in order to further reduce the cost, the GaN substrate having a small thickness may be cracked when subjected to working such as polishing. Even if the GaN substrate remains uncracked, an epitaxialwafer comprising the GaN substrate and epitaxial layers may be remarkably warped when the epitaxial layers are formed on the GaN substrate. In this case, photolithography or the like cannot be performed in order to form an electrode on this epitaxialwafer, and the epitaxialwafer cannot be applied to a semiconductor device.